I propose an in-vitro study of the way in which specific deformability properties of the human red cell influence the ease with which individual red cells flow through capillaries. In this experiement, overall cellular deformability will be characterized by the transit time of the cell -- the time required for a cell to travel a single 5-7 Mum diameter capillary pore under the action of a constant pressure drop across the pore. I hypothesize that changes in certain extrinsic and intrinsic cellular properties can influence dramatically the cellular transit time in-vitro and, thus, would influence or "control" capillary flow in-vivo. The particular properties to be studied are (1) cell volume (for a given surface area), (2) cell shape (for a given volume and surface area, (3) cell membrane elasticity, (4) cell membrane viscosity and (5) hemoglobin viscosity. Changes in these individual properties will be produced by physical (e.g. heat treatment) and chemical (e.g. osmolarity) means. My long term objective is to increase our basic understanding of the relation between cell properties and capillary flow and to achieve greater insight into those diseases, like sickle-cell anemia and diabetes, with consequences at the microvascular level.